The cylindrical roller bearing has a plurality of cylindrical rollers rollably disposed between a raceway surface formed substantially centrally on an outer periphery of the inner ring and a raceway surface formed substantially centrally on an inner periphery of the outer ring.
The above cylindrical rollers are circumferentially retained at predetermined intervals by means of the cage interposed between the outer ring and the inner ring. Specifically, the above-described cage forms a plurality of pockets defined by a pair of annular portions opposed to each other and spaced a predetermined distance apart in the axial direction and cage bars extending axially to interconnect the pair of annular portions. The cage is adapted to retain the respective cylindrical rollers in the pockets so that the cylindrical rollers are retained at the predetermined intervals in the circumferential direction.
One of the known examples of the above cylindrical roller bearing is a one wherein guide portions capable of sliding contact with axially opposite ends of the inner periphery of the outer ring are provided on outer peripheries of the respective annular portions, so that the guide portions may guide the rotation of the cage by making sliding contact with the inner periphery of the outer ring during the rotation of the bearing (see, for example, Japanese Unexamined Patent Publication No. 159535/1999).
However, the above-described cylindrical roller bearing has the following problem. Since the cylindrical rollers are confined between the guide portions, grease or lubricating oil supplied onto the raceway surface of the outer ring or a lubricant such as oil supplied by oil-air lubrication is restrained, by the guide portions, from migrating axially outwardly of the cage. Therefore, the lubricant is apt to accumulate on the raceway surface and the accumulated lubricant gives rolling resistance to the cylindrical rollers, so that the bearing is prone to arise in bearing torque.
For the prevention of lubricant accumulation, it may be contemplated to produce a gap between the guide portion and the raceway surface by reducing the overall size of the guide portion, thereby allowing the lubricant to migrate through this gap. However, this approach involves another problem that the cage is prone to eccentricity due to the presence of the gap, disabling the guide portions to guide the rotation of the cage favorably.